The nature of the cell membrane receptor to which glucose and other monosaccharides are bound prior to and during their penetration into the cell is being studied. Phlorizin and a number of organically synthesized compounds related in structure have been found to be preferentially bound to this glucose transport receptor and thus inhibit the membrane penetration of sugars into mammalian cells. This characteristic binding property serves as a fingerprint which can be used to identify the membrane component in attempts to isolate it from various mammalian cell membranes. Radioactively labeled phlorizin and phlorizin analogs, including some potential photo-affinity labelling derivatives, have been synthesized to selectively tag the receptor molecule(s) and the juxtaposed components making up the active center of the sugar carrier system. These findings are being correlated with the results of affinity chromatography studies designed to selectively isolate the sugar carrier system from fractions of solubilized cell membranes. Phlorizin and some of these derivatives are also being employed to investigate the relationship of the various glycosidases (sucrase, lactase, etc.) in the intestinal brush border membrane with the membrane machinery responsible for the uptake of hexose into the epithelial cell.